In Iowa, Sanders’s favorability rating has grown from about 35 percent
at the start of the year to 60 percent now. And in New Hampshire, it has
improved from around 45 percent to 65 percent. Some of that is from
improved name recognition, but Sanders’s unfavorable ratings haven’t
increased even as he’s become better known, remaining at about 10
percent in each state.

From 35% to 60% in Iowa, from 45% to 65% in New Hampshire, both since February. These are excellent numbers. Now the cause, as Silver sees it. The article begins with the idea he wants to debunk:

“The recent rise of Bernie Sanders,” wrote
Vox’s Jonathan Allen last week, “points as much to [Hillary] Clinton’s
vulnerability as Sanders’s strength.” Allen went on to argue that Joe
Biden should run for president. “The Sanders surge shows that Democratic
activists want an alternative to Clinton,” he explained.

We’ve seen this idea before. For at least a year, journalists have been urging, sometimes almost begging, Biden to enter the race. The more elaborate versions of the idea liken the 2016 campaign to 1968,
a year in which the incumbent president, Lyndon B. Johnson, withdrew
after the liberal, anti-war candidate Eugene McCarthy finished a close
second in the New Hampshire primary. The nomination was eventually won
by Johnson’s vice president, Hubert Humphrey, after Robert F. Kennedy
(who had entered the race after New Hampshire) was assassinated. In the
2016 narrative, Clinton is Johnson, Sanders is McCarthy and Biden is
some composite of Kennedy and Humphrey.

But these comparisons suffer from a fatal flaw. Unlike LBJ, who (mostly because of the Vietnam War) had approval ratings only in the mid-50s or low 60s
among Democrats during the 1968 campaign, Hillary Clinton is beloved by
voters in her party. In national polls, her favorability ratings among
Democrats usually exceed 80 percent.

Thus:

The Bernie Sanders surge, in other words, has a lot more to do with
Bernie Sanders than with Hillary Clinton. More specifically, it has to
do with his left-populist politics. We’re going to break some news here:
It turns out that some Democrats are really liberal, and they like a
really liberal candidate like Sanders. Right now, Sanders is winning about half the support of white liberal Democrats,
but little support from other groups within the party. That works out
to around 25 or 30 percent of the vote in Iowa and New Hampshire but
more like 15 percent among Democrats nationally.

But Silver's news for Sanders is not all good.

The Dark Side of Silver's Silver Lining

Silver is concerned (on Sanders' behalf) that this result — potential wins in Iowa and New Hampshire, each with a rich lode of white liberal Democrats — may be as good as it gets for him. In other words, that Sanders may win in Iowa and New Hampshire and lose everywhere else.

Here's what that "white liberal Democrat" population looks like, state by state:

Click to enlarge.

From a different FiveThirtyEight piece (the one containing the graphic above), Silver writes this:

Sanders, who has sometimes described himself as a socialist, isn’t
likely to do so well with moderate Democrats, of course. That’s a
problem for him, since a thin majority of Democrats still identify as
moderate or conservative rather than liberal. But Sanders has a few
things working in his favor. The share of liberal Democrats is increasing
— pretty rapidly, in fact — and those Democrats who turn out to vote in
the primaries tend to be more liberal than Democrats overall.

What’s received less attention is that Sanders has so far made very little traction with non-white Democrats. The most recent CNN poll found his support at just 9 percent among non-white Democrats, while the latest Fox News poll
had him at only 5 percent among African-American Democrats. (Fox News
did not provide crosstabs for Hispanics or other minority groups.)

So Sanders (and Warren) seem to be creating new "liberals" — meaning the Warren kind, not the Obama kind —pretty rapidly. That's good news. But without minority support, he's going to have some trouble. And that's fixable. I've heard from a number of sources that the Sanders camp is aware of this. More as it develops.

In the meantime...

Sanders Takes It to Clinton

This is one way to use the advantage you have — draw clear lines. Two reports, each from The Hill, around the same event, an impromptu Sanders press conference. First (my emphasis):

Sanders uses Clinton visit to draw contrast

Sen. Bernie Sanders (I-Vt.) on Tuesday tried to highlight the differences between himself and fellow presidential candidate Hillary Clinton after she spoke with Senate Democrats during a weekly party lunch.

Sanders, who, like Clinton, is seeking the 2016 Democratic nomination, spoke with reporters at a stakeout normally reserved for leadership from both parties. He used the impromptu press conference to underscore differences the two have on a wide range of issues, including trade, the war on Iraq, climate change and national security.

Pointing to his vote against the North American Free Trade Agreement (NAFTA), Sanders said that he believes the trade deal has "been disastrous for American workers. ...Secretary of State Clinton I believe has a different view on that issue."

He also cited his vote against authorizing the war against Iraq. Clinton, who was a senator at the time, has faced criticism for her vote supporting the war, and she told reporters earlier this year that she "made a mistake."

On climate change, he touted his opposition to the Keystone pipeline adding that "I think Secretary Clinton has not been clear on her views on that issue."

The Hill used that last remark — on Keystone and climate change — for a second piece about that press conference:

Speaking with reporters in the Capitol, Sanders he took a leadership role in Democrats’ fight against the proposed Canada-to-Texas pipeline, while Clinton has been silent on the project.

“I have helped lead the opposition against the Keystone pipeline,” the Vermont senator said. “I think Secretary Clinton has not been clear on her views on that issue.”

In a later statement, Sanders added that he opposes Keystone “because of concerns about climate change.”...

Clinton’s silence on Keystone has been one of the top sticking points for environmentalists, who have mostly avoided endorsing her.

As secretary of state from 2009 to 2013, Clinton was one of the most senior Obama administration officials responsible for its review of TransCanada Corp.’s application to build the pipeline, a process that has stretched on for more than six years.

In 2010, she said the administration was “inclined” to approve the project. But she has been silent since then.

Sanders, meanwhile, has been actively courting environmentalists, and was ranked by the super PAC Climate Hawks Vote as the best senator on climate in the 2013-2014 session of Congress.

If the Sanders surge lasts through February, when the Iowa caucus and New Hampshire primary are held, he's viable through the start of next year at least. That's a full six months to present the Sanders case to the rest of the Democratic base. So far, so good, though I would think it would be good to start making that case soon.